1. Field of the Invention
This invention relates to electric arc furnace steelmaking systems and, in particular, to an apparatus and a method for increasing spatial coverage of chemical energy in such a furnace.
2. Description of the Related Art
There has been much advancement in the art of electric arc furnace steelmaking that has produced more efficient methods for producing steel products. Steelmakers have generally strived to increase the efficiency of processes used in steelmaking processes. In the quest for more efficient processes, higher amounts of energy are desired in electric arc furnaces during the various stages of the steelmaking processes. Those stages are generally divided in the heating/melting stage and one refining stage of molten material. Steelmakers generally increase efficiency in steelmaking processes by implementing energy in addition to electrical energy, in the form of chemical energy, for example. Those skilled in the art may use various methods to implement the chemical energy into process/combustion chambers. In order to increase efficiency in steelmaking processes, steelmakers generally aspire to increase spatial coverage of the chemical energy in electric arc furnaces.
Turning now to FIG. 1, a depiction of a prior art electric arc furnace 100 is illustrated. The electric arc furnace 100 generally comprises three portions: a roof 110, an upper shell 120, and a lower shell 130. The electrodes 150 can move up and down through the roof 110. The line 140 between the upper shell and the lower shell is called the split line. The upper shell 120 is generally made out of water-cooled steel or copper panels. The lower shell 130 is usually non-water-cooled refractory lined. The internal space delimited by the lower shell 130, the upper shell 120 and the roof 110 constitutes the vessel in which the process of steelmaking takes place. Steelmakers generally implement additional chemical energy to increase spatial energy coverage through the walls that constitutes the upper shell 120 of the electric arc furnace 100. The upper shell 120 of the electric arc furnace 100 is described in further detail below.
In addition to electrical energy, chemical energy is added to the upper shell 120 of the furnace 100. The chemical energy input may be introduced into the electric arc furnace 100 via dedicated lances or burners. The chemical energy input generally includes oxidant gas and/or hydrocarbon fuel (gaseous or liquid). The oxidant gas generally refers to oxygen-enriched air, with oxygen concentration being more than a predetermined percentage (e.g., 25%) by volume of oxygen. Those skilled in the art will appreciate that the term “oxygen” may include oxidant gas and/or oxygen enriched-gas.
Lances may be used to inject chemical energy into the electric arc furnace 100. One category of lances 260 is made of consumable pipe that may carry the desired gases to be added to the furnace 100. Such a lance is generally moved through the tunnel, which is closed generally by a movable slag door 240 associated with the furnace 100, and is either manually operated or operated by automated moving devices. Such a lance 260 is generally submerged either inside the molten pool bath or in the slag. Generally, lances 260 are inserted into the furnace 100 at predetermined angles. A typical insertion angle of the oxygen injected through consumable lances may be 10–15 degrees relative to horizontal reference. Another alternative is to use water-cooled lances 260 moving through the tunnel closed by the slag door 240 or through the sidewalls of the upper shell 120 of the furnace 100. In the case where oxygen is injected through the lance 260, the velocity of the oxygen jet produced by the lances 260 is generally supersonic velocity, and the vertical angle of the oxygen jet pointing down may be approximately 55 degrees relative to a horizontal reference.
The chemical energy injected by the lances 260 is traditionally limited to an oxygen injection and/or a carbon particulate injection. Some chemical injection systems also have the function of generating a flame through a burner. In some state-of-the-art embodiments, the flame shrouds the supersonic oxygen jet, insuring a longer travel distance at supersonic velocity. An example of this multi-function tool is the PyreJet, commercially promoted by Air Liquide, Inc. A description of the PyreJet and its implementation may be found in U.S. Pat. Nos. 4,622,007 and 5,599,375 or in “Further Advances in EAF Efficiency with PyreJet burner injection”, published in September 2001 by Steel Times International.
Turning now to FIG. 2, a cross-section diagram of the upper shell 120 of the prior art electric arc furnace 100 is illustrated. The portion of the electric arc furnace 100 illustrated in FIG. 2 is one of an AC furnace. The three electrodes 150 are shown. The depiction in FIG. 2 illustrates a sidewall 220 enclosing the upper shell 120 of the electric arc furnace 100, which delimits the interior of the electric arc furnace. The movable slag door 240 is used to evacuate the furnace 100 of slag formed during the steelmaking process. Also, the opening of the slag door 240 is used to facilitate the sampling of steel temperature and the sampling for carbon content analysis. Additionally, the slag door 240 may be opened to introduce various fluids or particles in the reaction vessel, including oxygen and/or carbon particulates by lances 260 as described above. Various injection points or openings 250 may be installed within in the sidewalls 220 in order to allow for injection of fluids or particles, in order to introduce chemical energy inside the electric arc furnace 100 through the sidewalls 220. The injection points 250 allow for the injection of fluids or materials, which generally results in affected areas 230 where the chemical energy may be concentrated. For instance, if the injected fluids are fuel and oxygen, the affected area 230 may be defined by the flame envelope. Typically, utilizing state of the art technology, the affected areas 230 are generally small. There is a desire to produce a larger affected area 230 to introduce more efficient chemical energy implementation into the electric arc furnace 100 and increase the spatial coverage of such implementation.
To maintain efficiency, the number of openings in the sidewall 220 is generally limited. Typically, the number of openings in the sidewall 220 for use as injection points 250 is limited to three to five openings. The illustration in FIG. 2 depicts injection points 250 in the sidewall 220 and the slag door 240. If a DC furnace were used, those skilled in the art would appreciate that lance or burners used would generally point away from the central electrodes. As described above, one problem associated with state-of-the art implementation that the effect of chemical energy input in the furnace 100 is very spatially limited.
In using burners 260 to introduce chemical energy, typically, the burners 260 will preheat the scrap positioned within the electric arc furnace 100 interior in front of the said burners. During the supersonic lancing mode, the oxygen stream initially cuts the preheated scrap, penetrates through the preheated scrap, leading to the formation of a hole which provides a path to the molten liquid pool in the combustion chamber, thereby allowing oxygen to reach the molten bath. The oxygen may then engage in metallurgical reactions. However, this implementation only provides these benefits in a limited, localized region. In other words, this reaction is only spatially limited to the affected areas 230 illustrated in FIG. 2.
In introducing chemical energy using current methodology, energy release resulting from the injections of fuel and/or oxygen space is only effective directly in front of the burner 260. However, immediately on the sides of the burner 260, scrap is not adequately affected by the chemical energy. As a result, scrap between injections points 250 may not become adequately melted. Since the ultimate goal of the process is to melt the entire scrap content of the furnace 100, the inefficiency in the current processes will cause the entire process to be performed at a much slower rate. Furthermore, the improperly melted scrap may fall in the melted steel bath and will cool down the molten bath. This undesirable effect is generally referred to as a phenomenon known as late “cave in.”
In current steelmaking processes, particulates are also injected into the electric arc furnace 100 in order to enhance process efficiency. Typically, the particulates include carbon particles since additional energy is release when the injected carbon is combusted. Furthermore, the injected carbon is used to promote foaming slag reactions. The foaming slag generally surrounds the electric arc, thereby providing protection for various internal parts of the furnace 100 from arc direct radiations. Another benefit of introducing carbon injection into the combustor is a noticeable reduction in FeO, which consequently increases metallic yield. One solution per state-of-the-art technology is to inject carbon through the opened slag door 240. This solution, however, has reached its limits. In order to perform the carbon injection via the opened slag door 240, manual operations are generally required, which can be dangerous. Traditional movable manipulators used via the opened slag door 240 are generally very high maintenance parts. Also, foaming is limited to the area in front of the furnace door. With such practices, oxidized slag is immediately and continuously lost through the door immediately decreasing the yield of the process.
An alternative solution is to inject carbon particulates through the furnace sidewalls 220. This solution is provided in FIG. 3 and accompanying description below. Turning now to FIG. 3, a diagram illustrating the prior art method for injecting carbon is illustrated. Such injection generally requires a panel 310, which is usually water-cooled, made out of copper and embedded in water-cooled elements 340, composing the upper shell 120. FIG. 3 illustrates a prior art panel 310 that is used to perform carbon injection illustrated by the line/vector 320. Additionally, a lance or burner 330 is illustrated to inject oxygen, illustrated by the line/vector 350. The lance or burner is directly injected into the furnace interior. Various water-cooled elements 340 are surrounding the panel 310 and those elements compose the furnace sidewalls 220 of the upper shell 120. The portion of the panel 310 facing the inside of the upper shell 120 is aligned with the water-cooled elements 340. Additionally, the panel 310 is positioned atop the refractory line 360. The carbon injection and the oxygen injection, shown by the lines 320 and 350, are directed to the upper surface of the molten metal pool bath 315.
Various problems are associated with the implementation of the current methodology. For example, the carbon injection illustrates a line that is deflected (see line 320) from the upper surface of the molten metal pool bath 315 indicating that the penetration of the carbon injection may not be performed efficiently. As described above, the carbon injection is limited due to the bouncing of the injection material, as shown by the vector 320. Therefore, a portion of the injected particulates will bounce from the slag upper surface 316 and/or the molten metal pool bath surface 315 and will be entrained in the exhaust 540, exiting the furnace 100. Therefore, current practices generally lead directly to a partial waste of the carbon. In addition, un-burned carbon will eventually be dissociated into CO. The increase in CO level in the exhaust gas is an undesirable effect. Additionally, the increase in the CO level may generate combustion, explosion, increase in temperature, leading to problems in the water-cooled duct and bag-house.
In electric arc furnace steelmaking processes, it is desirable to inject carbon as early as possible during the heating/melting stage. With current layout and practices, steelmakers have to delay the carbon injection. One reason for such delay is due to a phenomenon known as “scrap cave-in.” The injection of carbon initiates the foaming reactions of the slag, known as foaming slag. If the carbon injection is introduced before the scrap is completely melted, the foaming slag may lead to slag-coating that may attach to un-melted scraps, preventing them from correct melting. This un-melted scrap may eventually form skulls sticking to the furnace walls. During the refining period, the skulls will eventually cave-in, which may cause strong, uncontrolled endothermic reactions. These reactions can be dangerous and may damage portions of the furnace 100, including the electrodes 150. Such reactions interrupt the steelmaking process and cool down the bath. Furthermore, these reactions may generate additional CO in the exhaust gas, which may cause additional problems described above. As a consequence, using state-of-the-art technologies, the furnace operators generally delay the start of carbon injection.
Another solution to improve efficiency of furnace operations is to perform early oxygen injection. However, this early oxygen injection may generate locally high FeO content in the slag, strong mixing, and high temperature as the result of exothermic reactions. Simultaneous carbon injections may reduce the locally high FeO content due to the early oxygen injection process; however, operators are reluctant to perform early carbon injection using state-of-the-art technology because of highly undesirable consequences described above.
Yet another reason for reducing the time period when carbon is injected in the process is related to the location of the carbon injection in the furnace. Because carbon injectors are generally close to the position where the oxygen is injected (both being above the bath), strong agitation is locally created. The strong agitation, the high level of chemical reactions, and locally high temperature level results, generally lead to refractory erosion. Because of the refractory erosion, steelmakers generally reduce the time and the amount of carbon that is implemented in the steelmaking process. Therefore, such a limitation results in yet another loss of efficiency in steelmaking processes.
Steelmakers have attempted to inject the carbon directly through the refractory line 360. However, one problem associated with such a process is the resulting direct erosion of the brick locally. Additionally, existing devices for injection through the refractory walls 360 have a limited life span, requiring a special system of cooling and require a specific quality of refractory for installation. These steps also adversely affect process efficiency. Furthermore, performing maintenance upon various portions of the furnace 100 is a routing process in the steelmaking industry. The maintenance rate may be influenced by the design and practices of the chemical energy injection points. The maintenance rate is especially high for burners 330 and the pipes used to inject the carbon. Slag splashing of the burner/lance head 330 may cause plugging problems. As described above, various problems and inefficiencies are present due to the implementation of current methodology. A more efficient method and apparatus for performing chemical energy injections are desirable in the steelmaking industry.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.